Run a Distribution-Supplied Kernel on a KVM Linode

Updated Friday, March 3, 2017 by Linode

As of February, 2017, you can boot your Linode using your choice of Linode’s own kernel or the upstream kernel provided by a distribution’s maintainers. Booting with Linode’s kernel is enabled by default, but changing to the distro-supplied kernel is easy. This is useful if you’d like to enable specific kernel features, or if you’d prefer to handle kernel upgrades yourself.

Recommended Distributions

The steps in this section currently apply only to the distributions under Recommended in the Linode Manager’s Deploy an Image dropdown.

1. Shut down your Linode from the Linode Manager.

2. Click Edit to view a distribution’s configuration profile options:

   

3. Under Boot Settings is a Kernel dropdown menu. By default, this will be set to the latest Linode-supplied 64 bit kernel:

   

4. To switch to the distro’s default kernel, select GRUB 2 instead of the latest 64 bit.

   

5. Click Save Changes at the bottom of the page and reboot into the new kernel.

Once booted, you can verify the kernel information with uname:

uname -r

This should return something similar to:

4.8.13-1-ARCH

If you want to switch back to the Linode kernel at any time:

1. Shut down your Linode.
2. Select the latest 64 bit Linode kernel using the steps above.
3. Click Save Changes and reboot.

Caveats

CentOS 7 and Fedora ship with SELinux installed and enabled by default. When switching from the Linode kernel to the CentOS or Fedora kernel, SELinux must run a relabeling of the filesystem to boot. Systemd will then reboot the Linode and if you have Lassie enabled, you’ll be back at the login prompt shortly. If you do not have Lassie enabled, you will need to manually Reboot in the Linode Manager.

The relabel process is triggered by the empty /.autorelabel file.

[root@li901-254 ~]# ls -a /
.   .autorelabel  boot  etc   lib    lost+found  mnt  proc  run   srv  tmp  var
..  bin           dev   home  lib64  media       opt  root  sbin  sys  usr

We include a systemd unit and bash script in our CentOS 7 images to automatically create this file when the Linode kernel is booted. This will save you from needing to do it manually before rebooting into the upstream kernel. You’ll find the systemd unit file at /etc/systemd/system/selinuxfsrelabel.service, which calls /usr/local/bin/selinuxfsrelabel.sh.

Older Distributions

The steps in this section apply to distributions that are not found in the Recommended section when deploying an image.

At the time of this writing, these steps have been tested on:

• Arch Linux
• CentOS
• Debian
• Fedora 24
• Gentoo
• Ubuntu

Install the Distribution Provided Kernel

1. Ensure that your system is up to date using the distribution’s package manager.

2. Install the Linux kernel. The package name differs based on your distribution:

   Arch Linux

   pacman -S linux grub
   

   CentOS 7

   yum install kernel grub2
   

   Debian

   apt-get install linux-image-amd64 grub2
   

   Fedora

   dnf install kernel-core grub2
   

   Gentoo

   mkdir /boot/grub
   echo "GRUB_PLATFORMS=\"coreboot pc\"" >> /etc/portage/make.conf
   emerge --ask sys-boot/grub sys-kernel/gentoo-sources genkernel
   eselect kernel list
   eselect kernel set [# of new kernel]
   zcat /proc/config.gz > /usr/src/linux/.config
   genkernel --oldconfig all
   

   Ubuntu

   apt-get install linux-image-virtual grub2
   

   Note

   During the installation of grub you may be asked which disk image to install to. Since Linode provides the grub bootloader, the system need only provide the grub.cfg file, and you don’t need to install grub to your MBR.

   You’ll see the kernel and other components you just installed and generated in the /boot directory. For example:

   [root@centos7 ~]# ls /boot
   config-3.10.0-514.el7.x86_64
   grub
   grub2
   initramfs-0-rescue-4f09fa5fdd3642fa85221d7c11370603.img
   initramfs-3.10.0-514.el7.x86_64.img
   initramfs-3.10.0-514.el7.x86_64kdump.img
   initrd-plymouth.img
   symvers-3.10.0-514.el7.x86_64.gz
   System.map-3.10.0-514.el7.x86_64
   vmlinuz-0-rescue-4f09fa5fdd3642fa85221d7c11370603
   vmlinuz-3.10.0-514.el7.x86_64
   

Configure Grub

1. Open /etc/default/grub in a text editor and go to the line beginning with GRUB_CMDLINE_LINUX. Remove the word quiet if present, and add console=ttyS0,19200n8 net.ifnames=0. Leave the other entries in the line. For example, on CentOS you should have something similar to:

   GRUB_CMDLINE_LINUX="crashkernel=auto rhgb console=ttyS0,19200n8 net.ifnames=0"
   

2. Then add or change the following options to match the following example. There will be other variables in this file, but we are only concerned with these lines.

   /etc/default/grub

   12345

   GRUB_TERMINAL=serial GRUB_DISABLE_OS_PROBER=true GRUB_SERIAL_COMMAND="serial --speed=19200 --unit=0 --word=8 --parity=no --stop=1" GRUB_DISABLE_LINUX_UUID=true GRUB_GFXPAYLOAD_LINUX=text

3. Run the following command to prepare and update the bootloader:

   Arch and Gentoo

   grub-mkconfig -o /boot/grub/grub.cfg
   

   CentOS and Fedora

   mkdir /boot/grub
   ln -s /boot/grub2/grub.cfg /boot/grub/grub.cfg
   grub2-mkconfig -o /boot/grub/grub.cfg
   touch /.autorelabel
   

   Note

   The autorelabel command is necessary to queue the SELinux filesystem relabeling process when rebooting from the Linode kernel to the CentOS or Fedora kernel.

   Debian and Ubuntu

   update-grub
   

See Also

• Run a Distribution-Supplied Kernel
• Custom Compiled Kernel on Debian & Ubuntu
• Install and Configure NixOS on a Linode
• Install a Custom Distribution on a Linode
• Custom Compiled Kernel on CentOS 7

This guide is published under a CC BY-ND 4.0 license.